Method and apparatus for accessing channels in wireless LAN system

ABSTRACT

The present invention relates to a wireless communication system, and more specifically, to a method and apparatus for accessing channels in a wireless LAN system. The method for accessing channels in a station (STA) of the wireless communication system according to one embodiment of the present invention comprises the steps of transmitting a first frame of a channel access request to an access point (AP); and receiving, from the AP, a second frame related to whether access to the channel is allowed in response to the first frame, wherein the second frame can include a value for a channel access start offset with respect to the STA.

This application is a National Stage Entry of International ApplicationNo. PCT/KR2013/000875 filed Feb. 4, 2013, which claims priority to U.S.Provisional Application Nos. 61/594,359 filed Feb. 2, 2012 and61/602,554 filed Feb. 23, 2012, all of which are incorporated herein byreference.

TECHNICAL FIELD

The following description relates to a wireless communication systemand, more particularly, to a method and apparatus for accessing achannel in a wireless local area network (LAN) system.

BACKGROUND ART

Recently, with development of information communication technology,various wireless communication technologies have been developed. Amongothers, a wireless local area network (WLAN) enables wireless access tothe Internet using a portable terminal such as a personal digitalassistant (PDA), a laptop, a portable multimedia player (PMP) in a home,an enterprise or a specific service provision area based on radiofrequency technology.

In order to overcome limitations in communication rate which have beenpointed out as weakness of a WLAN, in recent technical standards, asystem for increasing network speed and reliability and extendingwireless network distance has been introduced. For example, in IEEE802.11n, multiple input and multiple output (MIMO) technology usingmultiple antennas in a transmitter and a receiver has been introduced inorder to support high throughput (HT) with a maximum data rate of 540Mbps or more, to minimize transmission errors, and to optimize datarate.

DISCLOSURE Technical Problem

As next-generation communication technology, machine-to-machine (M2M)communication technology has been discussed. Even in an IEEE 802.11 WLANsystem, technical standards supporting M2M communication have beendeveloped as IEEE 802.11ah. In M2M communication, a scenario in which asmall amount of data is communicated at a low rate may be considered inan environment in which many apparatuses are present.

Communication in a WLAN system is performed in a medium shared betweenall apparatuses. As in M2M communication, if the number of apparatusesis increased, in order to reduce unnecessary power consumption andinterference, a channel access mechanism needs to be more efficientlyimproved.

An object of the present invention devised to solve the problem lies inan improved channel access method and apparatus in a WLAN system.

The technical problems solved by the present invention are not limitedto the above technical problems and those skilled in the art mayunderstand other technical problems from the following description.

Technical Solution

The object of the present invention can be achieved by providing amethod of performing channel access at a station (STA) in a wirelesscommunication system, including transmitting a first frame related to achannel access request to an access point (AP), and receiving, from theAP, a second frame related to whether channel access is granted, inresponse to the first frame, wherein the second frame includes a valueof a channel access start offset for the STA

In another aspect of the present invention, provided herein is a methodof supporting channel access of a station (STA) in an access point (AP)of a wireless communication system, including receiving a first framerelated to a channel access request from the STA, and transmitting, tothe STA, a second frame related to whether channel access is granted, inresponse to the first frame, wherein the second frame includes a valueof a channel access start offset for the STA.

In another aspect of the present invention, provided herein is a station(STA) apparatus for performing channel access in a wirelesscommunication system, including a transceiver, and a processor, whereinthe processor is configured to transmit a first frame related to achannel access request to an access point (AP) and to receive, from theAP, a second frame related to whether channel access is granted, inresponse to the first frame, wherein the second frame includes a valueof a channel access start offset for the STA.

In another aspect of the present invention, provided herein is an accesspoint (AP) apparatus for supporting channel access of a station (STA) ina wireless communication system, including a transceiver, and aprocessor, wherein the processor is configured to receive a first framerelated to a channel access request from the STA and to transmit, to theSTA, a second frame related to whether channel access is granted, inresponse to the first frame, wherein the second frame includes a valueof a channel access start offset for the STA.

The embodiments of the present invention may have the followingfeatures.

The value of the channel access start offset may determine when channelaccess of the STA starts to be granted or when the STA wakes up againafter entering a sleep state.

The first frame may include at least one of an association identifier(AID) of the STA, a group ID assigned to the STA or uplink (UL)/downlink(DL) channel access request indicator information.

The second frame may include at least one of a timestamp,acknowledgement (ACK), granted channel access duration, a current accessgroup number, a current channel access group identifier, a next accessgroup start offset, a channel access start offset or a new STAidentifier.

The channel access start offset may be one of a downlink data receptionstart time of the STA, a start time of a channel access interval atwhich uplink data transmission of the STA is granted, a next beacontransmission time, a next beacon transmission start time for a groupcorresponding to the STA, or a channel access start time of a groupassigned to the STA.

The second frame may include response type information, and the responsetype information may indicate one of channel access allowance, channelaccess rejection or STA identifier reassignment.

The first frame may be one of a power save (PS)-Poll frame or a channelaccess request frame.

The second frame may be one of an ACK frame, an access control frame ora channel access response frame.

The first frame may be transmitted if the STA is switched from a sleepstate to an awake state.

Without receiving a beacon including a traffic indication map (TIM),transmission of the first frame is allowed in an awake state of the STA.

Whether channel access is granted may be determined by the AP based onwhether a page identifier of the STA matches a page identifier relatedto a current channel access interval.

The transmitting the first frame related to the channel access requestto the AP may be performed during a channel access interval during whichchannel access of a group, to which the identifier of the STA belongs,is not granted.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

According to the present invention, it is possible to provide animproved channel access method and apparatus in a WLAN system. Accordingto the present invention, it is possible to provide a channel accessmethod and apparatus for preventing power consumption and interference.

The effects of the present invention are not limited to theabove-described effects and other effects which are not described hereinwill become apparent to those skilled in the art from the followingdescription.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a diagram showing an exemplary structure of an IEEE 802.11system to which the present invention is applicable;

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable;

FIG. 3 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable;

FIG. 4 is a diagram showing an exemplary structure of a WLAN system;

FIG. 5 is a diagram illustrating a link setup process in a WLAN system;

FIG. 6 is a diagram illustrating a backoff process;

FIG. 7 is a diagram illustrating a hidden node and an exposed node;

FIG. 8 is a diagram illustrating request to send (RTS) and clear to send(CTS);

FIG. 9 is a diagram illustrating power management operation;

FIGS. 10 to 12 are diagrams illustrating operation of a station (STA)which receives a traffic indication map (TIM);

FIG. 13 is a diagram illustrating a group based association identifier(AID);

FIGS. 14 to 16 are diagrams showing examples of operation of an STA if agroup channel access interval is set;

FIG. 17 is a diagram showing examples of a channel access request(CA-REQ) frame format proposed by the present invention;

FIGS. 18 to 20 are diagrams showing examples of a channel accessresponse (CA-RSP) frame format proposed by the present invention;

FIGS. 21 to 34 are diagrams showing examples of improved channel accessoperation using first and second frames proposed by the presentinvention;

FIG. 35 is a flowchart illustrating a channel access method according toan example of the present invention; and

FIG. 36 is a block diagram showing the configuration of a wirelessapparatus according to one embodiment of the present invention.

BEST MODE

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description set forth below in connection withthe appended drawings is intended as a description of exemplaryembodiments and is not intended to represent the only embodiments bywhich the concepts explained herein can be practiced. The detaileddescription includes details for the purpose of providing anunderstanding of the present invention. However, it will be apparent tothose skilled in the art that these teachings may be implemented andpracticed without these specific details.

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered optional factors on the conditionthat there is no additional remark. As required, the individualconstituent components or characteristics need not be combined withother components or characteristics. Also, some constituent componentsand/or characteristics may be combined to implement the embodiments ofthe present invention. The order of operations to be disclosed in theembodiments of the present invention may be rearranged. Some componentsor characteristics of any embodiment may also be included in otherembodiments, or may be replaced with those of the other embodiments asnecessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to another format within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention and theimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802 system, a 3^(rd) Generation Project Partnership (3GPP) system, a3GPP Long Term Evolution (LTE) system, and a 3GPP2 system. Inparticular, the steps or parts, which are not described to clearlyreveal the technical idea of the present invention, in the embodimentsof the present invention may be supported by the above documents. Allterminology used herein may be supported by at least one of theabove-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, CDMA (CodeDivision Multiple Access), FDMA (Frequency Division Multiple Access),TDMA (Time Division Multiple Access), OFDMA (Orthogonal FrequencyDivision Multiple Access), SC-FDMA (Single Carrier Frequency DivisionMultiple Access), and the like. CDMA may be embodied using radiotechnology such as UTRA (Universal Terrestrial Radio Access) orCDMA2000. TDMA may be embodied with radio technology such as GSM (GlobalSystem for Mobile communications)/GPRS (General Packet RadioService)/EDGE (Enhanced Data Rates for GSM Evolution). OFDMA may beembodied with radio technology such as Institute of Electrical andElectronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE802-20, and E-UTRA (Evolved UTRA). For clarity, the followingdescription focuses on the IEEE 802.11 system. However, technicalfeatures of the present invention are not limited thereto.

Structure of WLAN System

FIG. 1 is a diagram showing an exemplary structure of an IEEE 802.11system to which the present invention is applicable.

An IEEE 802.11 structure may be composed of a plurality of componentsand a wireless local area network (WLAN) supporting station (STA)mobility transparent to a higher layer may be provided by interactionamong the components. A basic service set (BSS) may correspond to abasic component block in an IEEE 802.11 LAN. In FIG. 1, two BSSs (BSS1and BSS2) are present and each BSS includes two STAs (STA1 and STA2 areincluded in BSS1 and STA3 and STA4 are included in BSS2) as members. InFIG. 1, an ellipse indicating the BSS indicates a coverage area in whichSTAs included in the BSS maintains communication. This area may bereferred to as a basic service area (BSA). If an STA moves out of a BSA,the STA cannot directly communicate with other STAs in the BSA.

In an IEEE 802.11 LAN, a BSS is basically an independent BSS (IBSS). Forexample, the IBSS may have only two STAs. In addition, the simplest BSS(BSS1 or BSS2) of FIG. 1, in which other components are omitted, maycorrespond to a representative example of the IBSS. Such a configurationis possible when STAs can directly perform communication. In addition,such a LAN is not configured in advance but may be configured if a LANis necessary. This LAN may also be referred to as an ad-hoc network.

If an STA is turned on or off or if an STA enters or moves out of a BSS,the membership of the STA in the BSS may be dynamically changed. An STAmay join a BSS using a synchronization process in order to become amember of the BSS. In order to access all services of a BSS basedstructure, an STA should be associated with the BSS. Such associationmay be dynamically set and may include use of a distribution systemservice (DSS).

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In FIG. 2, adistribution system (DS), a distribution system medium (DSM) and anaccess point (AP) are added to the structure of FIG. 1.

In a LAN, a direct station-to-station distance may be restricted by PHYperformance. Although such distance restriction may be possible,communication between stations located at a longer distance may benecessary. In order to support extended coverage, a DS may beconfigured.

The DS means a structure in which BSSs are mutually connected. Morespecifically, the BSSs are not independently present as shown in FIG. 1but the BSS may be present as an extended component of a networkincluding a plurality of BSSs.

The DS is a logical concept and may be specified by characteristics ofthe DSM. In IEEE 802.11 standards, a wireless medium (WM) and a DSM arelogically distinguished. Logical media are used for different purposesand are used by different components. In IEEE 802.11 standards, suchmedia are not restricted to the same or different media. Since pluralmedia are logically different, an IEEE 802.11 LAN structure (a DSstructure or another network structure) may be flexible. That is, theIEEE 802.11 LAN structure may be variously implemented and a LANstructure may be independently specified by physical properties of eachimplementation.

The DS provides seamless integration of a plurality of BSSs and provideslogical services necessary to treat an address to a destination so as tosupport a mobile apparatus.

The AP means an entity which enables associated STAs to access the DSvia the WM and has STA functionality. Data transfer between the BSS andthe DS may be performed via the AP. For example, STA2 and STA3 shown inFIG. 2 have STA functionality and provide a function enabling associatedSTAs (STA1 and STA4) to access the DS. In addition, since all APscorrespond to STAs, all APs may be addressable entities. An address usedby the AP for communication on the WM and an address used by the AP forcommunication on the DSM may not be equal.

Data transmitted from one of STAs associated with the AP to the STAaddress of the AP may always be received by an uncontrolled port andprocessed by an IEEE 802.1X port access entity. In addition, if acontrolled port is authenticated, transmission data (or frames) may betransmitted to the DS.

FIG. 3 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In FIG. 3,an extended service set (ESS) for providing wide coverage is added tothe structure of FIG. 2.

A wireless network having an arbitrary size and complexity may becomposed of a DS and BSSs. In an IEEE 802.11 system, such a network isreferred to as an ESS network. The ESS may correspond to a set of BSSsconnected to one DS. However, the ESS does not include the DS. The ESSnetwork appears as an IBSS network at a logical link control (LLC)layer. STAs included in the ESS may communicate with each other andmobile STAs may move from one BSS to another BSS (within the same ESS)transparently to the LLC layer.

In IEEE 802.11, relative physical locations of the BSSs in FIG. 3 arenot assumed and may be defined as follows. The BSSs may partiallyoverlap in order to provide consecutive coverage. In addition, the BSSsmay not be physically connected and a distance between BSSs is notlogically restricted. In addition, the BSSs may be physically located atthe same location in order to provide redundancy. In addition, one (ormore) IBSS or ESS network may be physically present in the same space asone (or more) ESS network. This corresponds to an ESS network type suchas a case in which an ad-hoc network operates at a location where theESS network is present, a case in which IEEE 802.11 networks physicallyoverlapped by different organizations are configured or a case in whichtwo or more different access and security policies are necessary at thesame location.

FIG. 4 is a diagram showing an exemplary structure of a WLAN system.FIG. 4 shows an example of an infrastructure BSS including a DS.

In the example of FIG. 4, BSS1 and BSS2 configure an ESS. In the WLANsystem, an STA operates according to a MAC/PHY rule of IEEE 802.11. TheSTA includes an AP STA and a non-AP STA. The non-AP STA corresponds toan apparatus directly handled by a user, such as a laptop or a mobilephone. In the example of FIG. 4, STA1, STA3 and STA4 correspond to thenon-AP STA and STA2 and STA5 correspond to the AP STA.

In the following description, the non-AP STA may be referred to as aterminal, a wireless transmit/receive unit (WTRU), a user equipment(UE), a mobile station (MS), a mobile terminal or a mobile subscriberstation (MSS). In addition, the AP may correspond to a base station(BS), a Node-B, an evolved Node-B (eNB), a base transceiver system (BTS)or a femto BS.

Link Setup Process

FIG. 5 is a diagram illustrating a general link setup process.

In order to establish a link with respect to a network and perform datatransmission and reception, an STA discovers the network, performsauthentication, establishes association and performs an authenticationprocess for security. The link setup process may be referred to as asession initiation process or a session setup process. In addition,discovery, authentication, association and security setup of the linksetup process may be collectively referred to as an association process.

An exemplary link setup process will be described with reference to FIG.5.

In step S510, the STA may perform a network discovery operation. Thenetwork discovery operation may include a scanning operation of the STA.That is, the STA discovers the network in order to access the network.The STA should identify a compatible network before participating in awireless network and a process of identifying a network present in aspecific area is referred to as scanning.

The scanning method includes an active scanning method and a passivescanning method.

In FIG. 5, a network discovery operation including an active scanningprocess is shown. In active scanning, the STA which performs scanningtransmits a probe request frame while moving between channels and waitsfor a response thereto, in order to detect which AP is present. Aresponder transmits a probe response frame to the STA, which transmittedthe probe request frame, as a response to the probe request frame. Theresponder may be an STA which lastly transmitted a beacon frame in a BSSof a scanned channel. In the BSS, since the AP transmits the beaconframe, the AP is the responder. In the IBSS, since the STAs in the IBSSalternately transmit the beacon frame, the responder is not fixed. Forexample, the STA which transmits the probe request frame on a firstchannel and receives the probe response frame on the first channelstores BSS related information included in the received probe responseframe, moves to a next channel (e.g., a second channel) and performsscanning (probe request/response transmission/reception on the secondchannel) using the same method.

Although not shown in FIG. 5, a scanning operation may be performedusing a passive scanning method. In passive scanning, the STA whichperforms scanning waits for a beacon frame while moving betweenchannels. The beacon frame is a management frame in IEEE 802.11 and isperiodically transmitted in order to indicate presence of a wirelessnetwork and to enable the STA, which performs scanning, to discover andparticipate in the wireless network. In the BSS, the AP is responsiblefor periodically transmitting the beacon frame. In the IBSS, the STAsalternately transmit the beacon frame. The STA which performs scanningreceives the beacon frame, stores information about the BSS included inthe beacon frame, and records beacon frame information of each channelwhile moving to another channel. The STA which receives the beacon framemay store BSS related information included in the received beacon frame,move to a next channel and perform scanning on the next channel usingthe same method.

Active scanning has delay and power consumption less than those ofpassive scanning.

After the STA has discovered the network, an authentication process maybe performed in step S520. Such an authentication process may bereferred to as a first authentication process to be distinguished from asecurity setup operation of step S540.

The authentication process includes a process of, at the STA,transmitting an authentication request frame to the AP and, at the AP,transmitting an authentication response frame to the STA in responsethereto. The authentication frame used for authenticationrequest/response corresponds to a management frame.

The authentication frame may include information about an authenticationalgorithm number, an authentication transaction sequence number, astatus code, a challenge text, a robust security network (RSN), a finitecyclic group, etc. The information may be examples of informationincluded in the authentication request/response frame and may bereplaced with other information. The information may further includeadditional information.

The STA may transmit the authentication request frame to the AP. The APmay determine whether authentication of the STA is allowed, based on theinformation included in the received authentication request frame. TheAP may provide the STA with the authentication result via theauthentication response frame.

After the STA is successfully authenticated, an association process maybe performed in step S530. The association process includes a processof, at the STA, transmitting an association request frame to the AP and,at the AP, transmitting an association response frame to the STA inresponse thereto.

For example, the association request frame may include information aboutvarious capabilities, beacon listen interval, service set identifier(SSID), supported rates, RSN, mobility domain, supported operatingclasses, traffic indication map (TIM) broadcast request, interworkingservice capability, etc.

For example, the association response frame may include informationabout various capabilities, status code, association ID (AID), supportedrates, enhanced distributed channel access (EDCA) parameter set,received channel power indicator (RCPI), received signal to noiseindicator (RSNI), mobility domain, timeout interval (associationcomeback time), overlapping BSS scan parameter, TIM broadcast response,QoS map, etc.

This information is purely exemplary information included in theassociation request/response frame and may be replaced with otherinformation. This information may further include additionalinformation.

After the STA is successfully authenticated, a security setup processmay be performed in step S540. The security setup process of step S540may be referred to as an authentication process through a robustsecurity network association (RSNA) request/response. The authenticationprocess of step S520 may be referred to as the first authenticationprocess and the security setup process of step S540 may be simplyreferred to as an authentication process.

The security setup process of step S540 may include a private key setupprocess through 4-way handshaking of an extensible authenticationprotocol over LAN (EAPOL) frame. In addition, the security setup processmay be performed according to a security method which is not defined inthe IEEE 802.11 standard.

Evolution of WLAN

As a technical standard recently established in order to overcomelimitations in communication speed in a WLAN, IEEE 802.11n has beendevised. IEEE 802.11n aims at increasing network speed and reliabilityand extending wireless network distance. More specifically, IEEE 802.11nis based on multiple input and multiple output (MIMO) technology usingmultiple antennas in a transmitter and a receiver in order to supporthigh throughput (HT) with a maximum data rate of 540 Mbps or more, tominimize transmission errors, and to optimize data rate.

As WLANs have come into widespread use and applications using the samehave been diversified, recently, there is a need for a new WLAN systemsupporting throughput higher than a data rate supported by IEEE 802.11n.A next-generation WLAN system supporting very high throughput (VHT) is anext version (e.g., IEEE 802.11ac) of the IEEE 802.11n WLAN system andis an IEEE 802.11 WLAN system newly proposed in order to support a datarate of 1 Gbps or more at a MAC service access point (SAP).

The next-generation WLAN system supports a multi-user MIMO (MU-MIMO)transmission scheme by which a plurality of STAs simultaneously accessesa channel in order to efficiently use a radio channel. According to theMU-MIMO transmission scheme, the AP may simultaneously transmit packetsto one or more MIMO-paired STAs.

In addition, support of a WLAN system operation in a whitespace is beingdiscussed. For example, introduction of a WLAN system in a TV whitespace(WS) such as a frequency band (e.g., 54 to 698 MHz) in an idle state dueto digitalization of analog TVs is being discussed as the IEEE 802.11afstandard. However, this is only exemplary and the whitespace may beincumbently used by a licensed user. The licensed user means a user whois allowed to use a licensed band and may be referred to as a licenseddevice, a primary user or an incumbent user.

For example, the AP and/or the STA which operate in the WS shouldprovide a protection function to the licensed user. For example, if alicensed user such as a microphone already uses a specific WS channelwhich is a frequency band divided on regulation such that a WS band hasa specific bandwidth, the AP and/or the STA cannot use the frequencyband corresponding to the WS channel in order to protect the licenseduser. In addition, the AP and/or the STA must stop use of the frequencyband if the licensed user uses the frequency band used for transmissionand/or reception of a current frame.

Accordingly, the AP and/or the STA should perform a procedure ofdetermining whether a specific frequency band in a WS band is available,that is, whether a licensed user uses the frequency band. Determiningwhether a licensed user uses a specific frequency band is referred to asspectrum sensing. As a spectrum sensing mechanism, an energy detectionmethod, a signature detection method, etc. may be used. It may bedetermined that the licensed user uses the frequency band if receivedsignal strength is equal to or greater than a predetermined value or ifa DTV preamble is detected.

In addition, as next-generation communication technology,machine-to-machine (M2M) communication technology is being discussed.Even in an IEEE 802.11 WLAN system, a technical standard supporting M2Mcommunication has been developed as IEEE 802.11ah. M2M communicationmeans a communication scheme including one or more machines and may bereferred to as machine type communication (MTC). Here, a machine meansan entity which does not require direct operation or intervention of aperson. For example, a device including a mobile communication module,such as a meter or a vending machine, may include a user equipment suchas a smart phone which is capable of automatically accessing a networkwithout operation/intervention of a user to perform communication. M2Mcommunication includes communication between devices (e.g.,device-to-device (D2D) communication) and communication between a deviceand an application server. Examples of communication between a deviceand a server include communication between a vending machine and aserver, communication between a point of sale (POS) device and a serverand communication between an electric meter, a gas meter or a watermeter and a server. An M2M communication based application may includesecurity, transportation, health care, etc. If the characteristics ofsuch examples are considered, in general, M2M communication shouldsupport transmission and reception of a small amount of data at a lowrate in an environment in which very many apparatuses are present.

More specifically, M2M communication should support a larger number ofSTAs. In a currently defined WLAN system, it is assumed that a maximumof 2007 STAs is associated with one AP. However, in M2M communication,methods supporting the case in which a larger number of STAs (about6000) are associated with one AP are being discussed. In addition, inM2M communication, it is estimated that there are many applicationssupporting/requiring a low transfer rate. In order to appropriatelysupport the low transfer rate, for example, in a WLAN system, the STAmay recognize presence of data to be transmitted thereto based on atraffic indication map (TIM) element and methods of reducing a bitmapsize of the TIM are being discussed. In addition, in M2M communication,it is estimated that there is traffic having a very longtransmission/reception interval. For example, in electricity/gas/waterconsumption, a very small amount of data is required to be exchanged ata long period (e.g., one month). In a WLAN system, although the numberof STAs associated with one AP is increased, methods of efficientlysupporting the case in which the number of STAs, in which a data frameto be received from the AP is present during one beacon period, is verysmall are being discussed.

WLAN technology has rapidly evolved. In addition to the above-describedexamples, technology for direct link setup, improvement of mediastreaming performance, support of fast and/or large-scale initialsession setup, support of extended bandwidth and operating frequency,etc. is being developed.

Medium Access Mechanism

In a WLAN system according to IEEE 802.11, the basic access mechanism ofmedium access control (MAC) is a carrier sense multiple access withcollision avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is alsoreferred to as a distributed coordination function (DCF) of IEEE 802.11MAC and employs a “listen before talk” access mechanism. According tosuch an access mechanism, the AP and/or the STA may perform clearchannel assessment (CCA) for sensing a radio channel or medium during apredetermined time interval (for example, a DCF inter-frame space(DIFS)) before starting transmission. If it is determined that themedium is in an idle state as the sensed result, frame transmissionstarts via the medium. If it is determined that the medium is in anoccupied state, the AP and/or the STA may set and wait for a delayperiod (e.g., a random backoff period) for medium access withoutstarting transmission and then attempt to perform frame transmission.Since several STAs attempt to perform frame transmission after waitingfor different times by applying the random backoff period, it ispossible to minimize collision.

In addition, the IEEE 802.11 MAC protocol provides a hybrid coordinationfunction (HCF). The HCF is based on the DCF and a point coordinationfunction (PCF). The PCF refers to a periodic polling method for enablingall reception AP and/or STAs to receive data frames using a pollingbased synchronous access method. In addition, the HCF has enhanceddistributed channel access (EDCA) and HCF controlled channel access(HCCA). The EDCA uses a contention access method for providing dataframes to a plurality of users by a provider and the HCCA uses acontention-free channel access method using a polling mechanism. Inaddition, the HCF includes a medium access mechanism for improvingquality of service (QoS) of a WLAN and may transmit QoS data both in acontention period (CP) and a contention free period (CFP).

FIG. 6 is a diagram illustrating a backoff process.

Operation based on a random backoff period will be described withreference to FIG. 6. If a medium is changed from an occupied or busystate to an idle state, several STAs may attempt data (or frame)transmission. At this time, a method of minimizing collision, the STAsmay select respective random backoff counts, wait for slot timescorresponding to the random backoff counts and attempt transmission. Therandom backoff count has a pseudo-random integer and may be set to oneof values of 0 to CW. Here, the CW is a contention window parametervalue. The CW parameter is set to CWmin as an initial value but may beset to twice CWmin if transmission fails (e.g., ACK for the transmissionframe is not received). If the CW parameter value becomes CWmax, datatransmission may be attempted while maintaining the CWmax value untildata transmission is successful. If data transmission is successful, theCW parameter value is reset to CWmin. CW, CWmin and CWmax values arepreferably set to 2^(n)−1 (n=0, 1, 2, . . . ).

If the random backoff process starts, the STA continuously monitors themedium while the backoff slots are counted down according to the setbackoff count value. If the medium is in the occupied state, countdownis stopped and, if the medium is in the idle state, countdown isresumed.

In the example of FIG. 6, if packets to be transmitted to the MAC ofSTA3 arrive, STA3 may confirm that the medium is in the idle stateduring the DIFS and immediately transmit a frame. Meanwhile, theremaining STAs monitor that the medium is in the busy state and wait.During a wait time, data to be transmitted may be generated in STA1,STA2 and STA5. The STAs may wait for the DIFS if the medium is in theidle state and then count down the backoff slots according to therespectively selected random backoff count values. In the example ofFIG. 6, STA2 selects a smallest backoff count value and STA1 selects alargest backoff count value. That is, the residual backoff time of STA5is less than the residual backoff time of STA1 when STA2 completesbackoff count and starts frame transmission. STA1 and STA5 stopcountdown and wait while STA2 occupies the medium. If occupancy of themedium by STA2 ends and the medium enters the idle state again, STA1 andSTA5 wait for the DIFS and then resume countdown. That is, after theresidual backoff slots corresponding to the residual backoff time arecounted down, frame transmission may start. Since the residual backofftime of STA5 is less than of STA1, STA5 starts frame transmission. IfSTA2 occupies the medium, data to be transmitted may be generated in theSTA5. At this time, STA4 may wait for the DIFS if the medium enters theidle state, perform countdown according to a random backoff count valueselected thereby, and start frame transmission. In the example of FIG.6, the residual backoff time of STA5 accidentally matches the randombackoff time of STA4. In this case, collision may occur between STA4 andSTA5. If collision occurs, both STA4 and STA5 do not receive ACK anddata transmission fails. In this case, STA4 and STA5 may double the CWvalue, select the respective random backoff count values and thenperform countdown. STA1 may wait while the medium is busy due totransmission of STA4 and STA5, wait for the DIFS if the medium entersthe idle state, and start frame transmission if the residual backofftime has elapsed.

Sensing Operation of STA

As described above, the CSMA/CA mechanism includes not only physicalcarrier sensing for directly sensing a medium by an AP and/or an STA butalso virtual carrier sensing. Virtual carrier sensing solves a problemwhich may occur in medium access, such as a hidden node problem. Forvirtual carrier sensing, MAC of a WLAN may use a network allocationvector (NAV). The NAV refers to a value of a time until a medium becomesavailable, which is indicated to another AP and/or STA by an AP and/oran STA, which is currently utilizing the medium or has rights to utilizethe medium. Accordingly, the NAV value corresponds to a period of timewhen the medium will be used by the AP and/or the STA for transmittingthe frame, and medium access of the STA which receives the NAV value isprohibited during that period of time. The NAV may be set according tothe value of the “duration” field of a MAC header of a frame.

A robust collision detection mechanism for reducing collision has beenintroduced, which will be described with reference to FIGS. 7 and 8.Although a transmission range may not be equal to an actual carriersensing range, for convenience, assume that the transmission range maybe equal to the actual carrier sensing range.

FIG. 7 is a diagram illustrating a hidden node and an exposed node.

FIG. 7(a) shows a hidden node, and, in this case, an STA A and an STA Bare performing communication and an STA C has information to betransmitted. More specifically, although the STA A transmits informationto the STA B, the STA C may determine that the medium is in the idlestate, when carrier sensing is performed before transmitting data to theSTA B. This is because the STA C may not sense transmission of the STA A(that is, the medium is busy). In this case, since the STA Bsimultaneously receives information of the STA A and the STA C,collision occurs. At this time, the STA A may be the hidden node of theSTA C.

FIG. 7(b) shows an exposed node and, in this case, the STA B transmitsdata to the STA A and the STA C has information to be transmitted to theSTA D. In this case, if the STA C performs carrier sensing, it may bedetermined that the medium is busy due to transmission of the STA B. Ifthe STA C has information to be transmitted to the STA D, since it issensed that the medium is busy, the STA C waits until the medium entersthe idle state. However, since the STA A is actually outside thetransmission range of the STA C, transmission from the STA C andtransmission from the STA B may not collide from the viewpoint of theSTA A. Therefore, the STA C unnecessarily waits until transmission ofthe STA B is stopped. At this time, the STA C may be the exposed node ofthe STA B.

FIG. 8 is a diagram illustrating request to send (RTS) and clear to send(CTS).

In the example of FIG. 7, in order to efficiently use a collisionavoidance mechanism, short signaling packet such as RTS and CTS may beused. RST/CTS between two STAs may be enabled to be overheard byperipheral STAs such that the peripheral STAs confirm informationtransmission between the two STAs. For example, if a transmission STAtransmits an RTS frame to a reception STA, the reception STA transmits aCTS frame to peripheral UEs to inform the peripheral UEs that thereception STA receives data.

FIG. 8(a) shows a method of solving a hidden node problem. Assume thatboth the STA A and the STA C attempt to transmit data to the STA B. Ifthe STA A transmits the RTS to the STA B, the STA B transmits the CTS tothe peripheral STA A and C. As a result, the STA C waits until datatransmission of the STA A and the STA B is finished, thereby avoidingcollision.

FIG. 8(b) shows a method of solving an exposed node problem. The STA Cmay overhear RTS/CTS transmission between the STA A and the STA B anddetermine that collision does not occur even when the STA C transmitsdata to another STA (e.g., the STA D). That is, the STA B transmits theRTS to all peripheral UEs and transmits the CTS only to the STA A havingdata to be actually transmitted. Since the STA C receives the RTS butdoes not receive the CTS of the STA A, it can be confirmed that the STAA is outside carrier sensing of the STA C.

Power Management

As described above, in a WLAN system, channel sensing should beperformed before an STA performs transmission and reception. When thechannel is always sensed, continuous power consumption of the STA iscaused. Power consumption in a reception state is not substantiallydifferent from power consumption in a transmission state andcontinuously maintaining the reception state imposes a burden on an STAwith limited power (that is, operated by a battery). Accordingly, if areception standby state is maintained such that the STA continuouslysenses the channel, power is inefficiently consumed without any specialadvantage in terms of WLAN throughput. In order to solve such a problem,in a WLAN system, a power management (PM) mode of the STA is supported.

The PM mode of the STA is divided into an active mode and a power save(PS) mode. The STA fundamentally operates in an active mode. The STAwhich operates in the active mode is maintained in an awake state. Theawake state refers to a state in which normal operation such as frametransmission and reception or channel scanning is possible. The STAwhich operates in the PS mode operates while switching between a sleepstate or an awake state. The STA which operates in the sleep stateoperates with minimum power and does not perform frame transmission andreception or channel scanning.

Since power consumption is reduced as the sleep state of the STA isincreased, the operation period of the STA is increased. However, sinceframe transmission and reception is impossible in the sleep state, theSTA may not unconditionally operate in the sleep state. If a frame to betransmitted from the STA, which operates in the sleep state, to the APis present, the STA may be switched to the awake state to transmit theframe. If a frame to be transmitted from the AP to the STA is present,the STA in the sleep state may not receive the frame and may not confirmthat the frame to be received is present. Accordingly, the STA needs toperform an operation for switching to the awake state according to aspecific period in order to confirm presence of the frame to betransmitted thereto (to receive the frame if the frame to be transmittedis present).

FIG. 9 is a diagram illustrating power management operation.

Referring to FIG. 9, an AP 210 transmits beacon frames to STAs within aBSS at a predetermined period (S211, S212, S213, S214, S215 and S216).The beacon frame includes a traffic indication map (TIM) informationelement. The TIM information element includes information indicatingthat buffered traffic for STAs associated with the AP 210 is present andthe AP 210 will transmit a frame. The TIM element includes a TIM used toindicate a unicast frame or a delivery traffic indication map (DTIM)used to indicate a multicast or broadcast frame.

The AP 210 may transmit the DTIM once whenever the beacon frame istransmitted three times. An STA1 220 and an STA2 222 operate in the PSmode. The STA1 220 and the STA2 222 may be switched from the sleep stateto the awake state at a predetermined wakeup interval to receive a TIMelement transmitted by the AP 210. Each STA may compute a time to switchto the awake state based on a local clock thereof. In the example ofFIG. 9, assume that the clock of the STA matches the clock of the AP.

For example, the predetermined awake interval may be set such that theSTA1 220 is switched to the awake state every beacon interval to receivea TIM element. Accordingly, the STA1 220 may be switched to the awakestate (S211) when the AP 210 first transmits the beacon frame (S211).The STA1 220 may receive the beacon frame and acquire the TIM element.If the acquired TIM element indicates that a frame to be transmitted tothe STA1 220 is present, the STA1 220 may transmit, to the AP 210, apower save-Poll (PS-Poll) frame for requesting frame transmission fromthe AP 210 (S221 a). The AP 210 may transmit the frame to the STA1 220in correspondence with the PS-Poll frame (S231). The STA1 220 whichcompletes frame reception is switched to the sleep state.

When the AP 210 secondly transmits the beacon frame, since anotherdevice access the medium and thus the medium is busy, the AP 210 may nottransmit the beacon frame at an accurate beacon interval and maytransmit the beacon frame at a delayed time (S212). In this case, theoperation mode of the STA1 220 is switched to the awake state accordingto the beacon interval but the delayed beacon frame is not received.Therefore, the operation mode of the STA1 220 is switched to the sleepstate again (S222).

When the AP 210 thirdly transmits the beacon frame, the beacon frame mayinclude a TIM element set to a DTIM. Since the medium is busy, the AP210 transmits the beacon frame at a delayed time (S213). The STA1 220 isswitched to the awake state according to the beacon interval and mayacquire the DTIM via the beacon frame transmitted by the AP 210. Assumethat the DTIM acquired by the STA1 220 indicates that a frame to betransmitted to the STA1 220 is not present and a frame for another STAis present. In this case, the STA1 220 may confirm that a frametransmitted thereby is not present and may be switched to the sleepstate again. The AP 210 transmits the beacon frame and then transmitsthe frame to the STA (S232).

The AP 210 fourthly transmits the beacon frame (S214). Since the STA1220 cannot acquire information indicating that buffered traffic thereforis present via reception of the TIM element twice, the wakeup intervalfor receiving the TIM element may be controlled. Alternatively, ifsignaling information for controlling the wakeup interval of the STA1220 is included in the beacon frame transmitted by the AP 210, thewakeup interval value of the STA1 220 may be controlled. In the presentexample, the STA1 220 may change switching of the operation state forreceiving the TIM element every beacon interval to switching of theoperation state every three beacon intervals. Accordingly, since theSTA1 220 is maintained in the sleep state when the AP 210 transmits thefourth beacon frame (S214) and transmits the fifth beacon frame (S215),the TIM element cannot be acquired.

When the AP 210 sixthly transmits the beacon frame (S216), the STA1 220may be switched to the awake state to acquire the TIM element includedin the beacon frame (S224). Since the TIM element is a DTIM indicatingthat a broadcast frame is present, the STA1 220 may not transmit thePS-Poll frame to the AP 210 but may receive a broadcast frametransmitted by the AP 210 (S234). The wakeup interval set in the STA2230 may be set to be greater than that of the STA1 220. Accordingly, theSTA2 230 may be switched to the awake state to receive the TIM element(S241), when the AP 210 fifthly transmits the beacon frame (S215). TheSTA2 230 may confirm that a frame to be transmitted thereto is presentvia the TIM element and transmits the PS-Poll frame to the AP 210 (S241a) in order to request frame transmission. The AP 210 may transmit theframe to the STA2 230 in correspondence with the PS-Poll frame (S233).

For PM management shown in FIG. 9, a TIM element includes a TIMindicating whether a frame to be transmitted to an STA is present and aDTIM indicating whether a broadcast/multicast frame is present. The DTIMmay be implemented by setting a field of the TIM element.

FIGS. 10 to 12 are diagrams illustrating operation of a station (STA)which receives a traffic indication map (TIM).

Referring to FIG. 10, an STA may be switched from a sleep state to anawake state in order to receive a beacon frame including a TIM from anAP and interpret the received TIM element to confirm that bufferedtraffic to be transmitted thereto is present. The STA may contend withother STAs for medium access for transmitting a PS-Poll frame and thentransmit the PS-Poll frame in order to request data frame transmissionfrom the AP. The AP which receives the PS-Poll frame transmitted by theSTA may transmit the frame to the STA. The STA may receive the dataframe and transmit an ACK frame to the AP. Thereafter, the STA may beswitched to the sleep state again.

As shown in FIG. 10, the AP may receive the PS-Poll frame from the STAand then operate according to an immediate response method fortransmitting a data frame after a predetermined time (e.g., a shortinter-frame space (SIFS)). If the AP does not prepare a data frame to betransmitted to the STA during the SIFS after receiving the PS-Pollframe, the AP may operate according to a deferred response method, whichwill be described with reference to FIG. 11.

In the example of FIG. 11, operation for switching the STA from thesleep state to the awake state, receiving a TIM from the AP, contendingand transmitting a PS-Poll frame to the AP is equal to that of FIG. 10.If the data frame is not prepared during the SIFS even when the APreceives the PS-Poll frame, the data frame is not transmitted but an ACKframe may be transmitted to the STA. If the data frame is prepared aftertransmitting the ACK frame, the AP may contend and transmit the dataframe to the STA. The STA may transmit the ACK frame indicating that thedata frame has been successfully received to the AP and may be switchedto the sleep state.

FIG. 12 shows an example in which the AP transmits the DTIM. The STAsmay be switched from the sleep state to the awake state in order toreceive the beacon frame including the DTIM element from the AP. The STAmay confirm that a multicast/broadcast frame will be transmitted via thereceived DTIM. The AP may immediately transmit data (that is, amulticast/broadcast frame) without PS-Poll frame transmission andreception after transmitting the beacon frame including the DTIM. TheSTAs may receive data in the awake state after receiving the beaconframe including the DTIM and may be switched to the sleep state againafter completing data reception.

TIM Structure

In the PM mode management method based on the TIM (or DTIM) protocoldescribed with reference to FIGS. 9 to 12, the STAs may confirm whethera data frame to be transmitted thereto is present via STA identificationincluded in the TIM element. The STA identification may be related to anassociation identifier (AID) assigned to the STA upon association withthe AP.

The AID is used as a unique identifier for each STA within one BSS. Forexample, in a current WLAN system, the AID may be one of values of 1 to2007. In a currently defined WLAN system, 14 bits are assigned to theAID in a frame transmitted by the AP and/or the STA. Although up to16383 may be assigned as the AID value, 2008 to 16383 may be reserved.

The TIM element according to an existing definition is not appropriatelyapplied to an M2M application in which a large number (e.g., more than2007) of STAs is associated with one AP. If the existing TIM structureextends without change, the size of the TIM bitmap is too large to besupported in an existing frame format and to be suitable for M2Mcommunication considering an application with a low transfer rate. Inaddition, in M2M communication, it is predicted that the number of STAs,in which a reception data frame is present during one beacon period, isvery small. Accordingly, in M2M communication, since the size of the TIMbitmap is increased but most bits have a value of 0, there is a need fortechnology for efficiently compressing the bitmap.

As an existing bitmap compression technology, a method of omitting 0which continuously appears at a front part of a bitmap and defining anoffset (or a start point) is provided. However, if the number of STAs inwhich a buffered frame is present is small but a difference between theAID values of the STAs is large, compression efficiency is bad. Forexample, if only frames to be transmitted to only two STAs respectivelyhaving AID values of 10 and 2000 are buffered, the length of thecompressed bitmap is 1990 but all bits other than both ends have a valueof 0. If the number of STAs which may be associated with one AP issmall, bitmap compression inefficiency is not problematic but, if thenumber of STAs is increased, bitmap compression inefficiencydeteriorates overall system performance.

As a method of solving this problem, AIDs may be divided into severalgroups to more efficiently perform data transmission. A specific groupID (GID) is assigned to each group. AIDs assigned based on the groupwill be described with reference to FIG. 13.

FIG. 13(a) shows an example of AIDs assigned based on a group. In theexample of FIG. 13(a), several bits of a front part of the AID bitmapmay be used to indicate the GID. For example, four DIDs may be expressedby the first two bits of the AID of the AID bitmap. If the total lengthof the AID bitmap is N bits, the first two bits (B1 and B2) indicate theGID of the AID.

FIG. 13(a) shows another example of AIDs assigned based on a group. Inthe example of FIG. 13(b), the GID may be assigned according to thelocation of the AID. At this time, the AIDs using the same GID may beexpressed by an offset and a length value. For example, if GID 1 isexpressed by an offset A and a length B, this means that AIDs of A toA+B−1 on the bitmap have GID 1. For example, in the example of FIG.13(b), assume that all AIDs of 1 to N4 are divided into four groups. Inthis case, AIDs belonging to GID 1 are 1 to N1 and may be expressed byan offset 1 and a length N1. AIDs belonging to GID2 may be expressed byan offset N1+1 and a length N2−N1+1, AIDs belonging to GID 3 may beexpressed by an offset N2+1 and a length N3−N2+1, and AIDs belonging toGID 4 may be expressed by an offset N3+1 and a length N4−N3+1.

If the AIDs assigned based on the group are introduced, channel accessis allowed at a time interval which is changed according to the GID tosolve lack of TIM elements for a large number of STAs and to efficientlyperform data transmission and reception. For example, only channelaccess of STA(s) corresponding to a specific group may be granted duringa specific time interval and channel access of the remaining STA(s) maybe restricted. A predetermined time interval at which only access ofspecific STA(s) is granted may also be referred to as a restrictedaccess window (RAW).

Channel access according to GID will be described with reference to FIG.13(c). FIG. 13(c) shows a channel access mechanism according to a beaconinterval if the AIDs are divided into three groups. At a first beaconinterval (or a first RAW), channel access of STAs belonging to GID 1 isgranted but channel access of STAs belonging to other GIDs is notgranted. For such implementation, the first beacon includes a TIMelement for AIDs corresponding to GID 1. A second beacon frame includesa TIM element for AIDs corresponding to GID 2 and thus only channelaccess of the STAs corresponding to the AIDs belonging to GID 2 isgranted during the second beacon interval (or the second RAW). A thirdbeacon frame includes a TIM element for AIDs corresponding to GID 3 andthus only channel access of the STAs corresponding to the AIDs belongingto GID 3 is granted during the third beacon interval (or the third RAW).A fourth beacon frame includes a TIM element for AIDs corresponding toGID 1 and thus only channel access of the STAs corresponding to the AIDsbelonging to GID 1 is granted during the fourth beacon interval (or thefourth RAW). Only channel access of the STAs corresponding to a specificgroup indicated by the TIM included in the beacon frame may be grantedeven in fifth and subsequent beacon intervals (or fifth and subsequentRAWs).

Although the order of GIDs allowed according to the beacon interval iscyclic or periodic in FIG. 13(c), the present invention is not limitedthereto. That is, by including only AID(s) belonging to specific GID(s)in the TIM elements, only channel access of STA(s) corresponding to thespecific AID(s) may be granted during a specific time interval (e.g., aspecific RAW) and channel access of the remaining STA(s) may not begranted.

The above-described group based AID assignment method may also bereferred to as a hierarchical structure of a TIM. That is, an entire AIDspace may be divided into a plurality of blocks and only channel accessof STA(s) corresponding to a specific block having a non-zero value(that is, STAs of a specific group) may be granted. A TIM having a largesize is divided into small blocks/groups such that the STA easilymaintains TIM information and easily manages blocks/groups according toclass, QoS or usage of the STA. Although a 2-level layer is shown in theexample of FIG. 13, a TIM of a hierarchical structure having two or morelevels may be constructed. For example, the entire AID space may bedivided into a plurality of page groups, each page group may be dividedinto a plurality of blocks, and each block may be divided into aplurality of sub-blocks. In this case, as an extension of the example ofFIG. 13(a), the first N1 bits of the AID bitmap indicate a paging ID(that is, a PID), the next N2 bits indicate a block ID, the next N3 bitsindicate a sub-block ID, and the remaining bits indicate the STA bitlocation in the sub-block.

In the following examples of the present invention, various methods ofdividing and managing STAs (or AIDs assigned to the STAs) on apredetermined hierarchical group basis are applied and the group basedAID assignment method is not limited to the above examples.

Improved Channel Access Method

If AIDs are assigned/managed based on a group, STAs belonging to aspecific group may use a channel only at a “group channel accessinterval (or RAW)” assigned to the group. If an STA supports an M2Mapplication, traffic for the STA may have a property which may begenerated at a long period (e.g., several tens of minutes or severalhours). Since such an STA does not need to be in the awake statefrequently, the STA may be in the sleep mode for g a long period of timeand be occasionally switched to the awake state (that is, the awakeinterval of the STA may be set to be long). An STA having a long wakeupinterval may be referred to as an STA which operates in a “long-sleeper”or “long-sleep” mode. The case in which the wakeup interval is set to belong is not limited to M2M communication and the wakeup interval may beset to be long according to the state of the STA or surroundings of theSTA even in normal WLAN operation.

If the wakeup interval is set, the STA may determine whether a localclock thereof exceeds the wakeup interval. However, since the localclock of the STA generally uses a cheap oscillator, an error probabilityis high. In addition, if the STA operates in long-sleep mode, the errormay be increased with time. Accordingly, time synchronization of the STAwhich occasionally wakes up may not match time synchronization of theAP. For example, although the STA computes when the STA may receive thebeacon frame to be switched to the awake state, the STA may not actuallyreceive the beacon frame from the AP at that timing. That is, due toclock drift, the STA may miss the beacon frame and such a problem mayfrequently occur if the STA operates in the long sleep mode.

FIGS. 14 to 16 are diagrams showing examples of operation of an STA if agroup channel access interval is set.

In the example of FIG. 14, STA3 may belong to group 3 (that is, GID=3),wake up at a channel access interval assigned to group 1 and performPS-Poll for requesting frame transmission from the AP. The AP whichreceives PS-Poll from the STA transmits an ACK frame to STA3. Ifbuffered data to be transmitted to STA3 is present, the AP may provideinformation indicating that data to be transmitted is present via theACK frame. For example, the value of a “More Data” field (or an MDfield) having a size of 1 bit included in the ACK frame may be set to 1(that is, MD=1) to indicate the above information.

Since a time when STA3 transmits PS-Poll belongs to the channel accessinterval for group 1, even if data to be transmitted to STA3 is present,the AP does not immediately transmit data after transmitting the ACKframe but transmits data to STA3 at a channel access interval (GID 3channel access of FIG. 14) assigned to group 3 to which STA3 belongs.

Since STA3 receives the ACK frame set to MD=1 from the AP, STA3continuously waits for transmission of data from the AP. That is, in theexample of FIG. 14, since STA3 cannot receive the beacon frameimmediately after waking up, STA3 transmits PS-Poll to the AP on theassumption that a time when STA3 wakes up corresponds to the channelaccess interval assigned to the group, to which STA3 belongs, accordingto computation based on the local clock thereof and data to betransmitted thereto is present. Alternatively, since STA3 operates inthe long-sleep mode, on the assumption that time synchronization is notperformed, if the data to be transmitted thereto is present, STA3 maytransmit PS-Poll to the AP in order to receive the data. Since the ACKframe received by STA3 from the AP indicates that data to be transmittedto STA3 is present, STA3 continuously waits for data reception under theassumption of the interval in which channel access thereof is granted.STA3 unnecessarily consumes power even when data reception is notallowed, until time synchronization is appropriately performed frominformation included in a next beacon frame.

In particular, if STA3 operates in the long-sleep mode, the beacon framemay frequently not be received, CCA may be performed even at the channelaccess interval, to which STA2 does not belong, thereby causingunnecessary power consumption.

Next, in the example of FIG. 15, the beacon frame is missed when the STAhaving GID 1 (that is, belonging to group 1) wakes up. That is, the STAwhich does not receive the beacon frame including the GID (or PID)assigned thereto is continuously in the awake state until the beaconframe including the GID (or PID) thereof is received. That is, althoughthe STA wakes up at channel access interval assigned thereto, the STAcannot confirm whether the GID (or PID) thereof is included in the TIMtransmitted via the beacon frame and thus cannot confirm whether thetiming corresponds to the channel access interval assigned to the groupthereof.

In the example of FIG. 15, the STA which is switched from the sleepstate to the awake state is continuously in the awake state until thefourth beacon frame including the GID (that is, GID 1) thereof isreceived after the first beacon frame has been missed, thereby causingunnecessary power consumption. As a result, after unnecessary powerconsumption, the STA may receive the beacon frame including GID 1 andthen may perform RTS transmission, CTS reception, data frametransmission and ACK reception.

FIG. 16 shows the case in which an STA wakes up at a channel accessinterval for another group. For example, the STA having GID 3 may wakeup at the channel access interval for GID 1. That is, the STA having GID3 unnecessarily consumes power until the beacon frame having the GIDthereof is received after waking up. If a TIM indicating GID 3 isreceived via a third beacon frame, the STA may recognize the channelaccess interval for the group thereof and perform data transmission andACK reception after CCA through RTS and CTS.

In the present invention, as described above, if group-based restrictedaccess is granted, an improved channel access method for preventing orreducing unnecessary power consumption of the STA is proposed. Inparticular, the channel access method proposed by the present inventionis applicable to the STA which operates in the long-sleep mode with ahigh probability that time synchronization with the AP is not performed.

In the present invention, if the STA switched from the sleep state tothe awake state transmits a frame (a first frame) including informationrelated to a channel access request to the AP, it is possible to preventunnecessary power consumption of the STA using the method of, at the AP,transmitting a frame (a second frame) including information indicatingwhether channel access is granted. A representative example of theinformation indicating whether channel access is granted may be timinginformation. The first frame may be an existing PS-Poll frame or a newframe (e.g., a channel access request frame) proposed by the presentinvention. The second frame may be an existing ACK frame, an accesscontrol frame or a new frame (e.g., a channel access response frame)proposed by the present invention. Hereinafter, the detailed examples ofthe present invention will be described.

According to the present invention, the STA which operates in thelong-sleep mode may perform access to a channel (that is, a downlink(DL) channel) from the AP to the STA and/or a channel (that is, anuplink (UL) channel) from the STA to the AP even when the beacon frameis not received (alternatively, even when the STA is not maintained inthe awake state until the beacon frame including the GID (or PID)thereof is received).

In this case, the STA may transmit the first frame to the AP to requestchannel access. The first frame may be a PS-Poll or channelaccess-request (CA-REQ) frame in the present invention. The presentinvention is not limited thereto and predetermined frames includinginformation described in the various examples of the present inventionmay be collectively referred to as the first frame.

The first frame may be transmitted at any time. That is, the first framemay be transmitted even when the STA cannot confirm whether DL datatransmitted thereto is present (e.g., even when the TIM is notreceived).

FIG. 17 is a diagram showing examples of a channel access request(CA-REQ) frame format proposed by the present invention.

In general, the frame may include an MAC header, a payload and a framecheck sequence (FCS). A certain frame may not have a payload part. Thefirst 2 octets (that is, 16 bits) of the MAC header corresponds to aframe control field. The frame control field may include a protocolversion field, a type field, a subtype field, a more data (MD) field,etc. The last 2 octet of the frame may be composed of the FCS.

As shown in FIG. 17(a), the CA-REQ frame may include an identifier (thatis, AID) of an STA and an identifier (that is, a BSSID) of an AP. TheAID is filled with the AID assigned to the STA and the BSSID may befilled with the identifier of the AP to which the STA transmits theCA-REQ frame. Information indicating whether the frame is a CA-REQ frameor not may be indicated using the type field and the subtype field ofthe frame control field. If the STA has UL data to be transmitted to theAP, the MD bit of the frame control field may be set to 1 to transmitthe CA-REQ frame. In this case, since the AID field includes the GID (orPID) (that is, the AP can confirm the GDI (or PID), to which the AIDbelongs, through the AID of the STA), the GID (or PID) field does notneed to be separately included in the CA-REQ frame.

In addition, the GID of the STA may be indicated by a hierarchical AIDstructure and a specific AID range within the AID range may be used toindicate the GID (see FIG. 13(a)). Accordingly, as shown in FIG. 17(b),the CA-REQ frame may be configured to separately include the AID fieldand the GID field.

As an additional example, as shown in FIG. 17(c), the CA-REQ frame mayfurther include a UL/DL indicator field in addition to the exemplaryframe structure of FIG. 17(a). The UL/DL indicator field may indicatewhether the CA-REQ frame is an access request for a DL channel or anaccess request for a UL channel.

The embodiments of the present invention using various examples of thefirst frame will be described in detail after describing the format ofthe second frame.

The AP which has received the first frame (e.g., PS-Poll or CA-REQframe) may transmit the second frame (e.g., ACK, access control orchannel access-response (CA-RSP) frame to the STA in response thereto.

FIGS. 18 to 20 are diagrams showing examples of a channel accessresponse (CA-RSP) frame format proposed by the present invention.

As shown in FIGS. 18 to 20, the examples of the CA-RSP frame may bedefined to include a frame control field, an AID field, a BSSID fieldand a FCS field. Information indicating whether the frame is a CA-RSPframe may be indicated using a type field and a subtype field of theframe control field. If the AP has DL data to be transmitted to the STA,the MD bit of the frame control field may be set to 1 to transmit theCA-RSP frame.

In addition, the CA-RSP frame may include informationdirectly/indirectly indicating whether channel access is granted to theSTA which does not receive the beacon frame after waking up (or the STAwhich transmits the first frame). This may be referred to as responseinformation.

In the example of FIG. 18, the response message may include timestampinformation (FIG. 18(a)), ACK information (FIG. 18(b)), channel accessstart offset (CASO) and granted channel access duration (GCAD)information (FIG. 18(c)) or current access group number (CAGN) and nextaccess group start offset (NAGSO) information (FIG. 18(d)). The lengthof each field (L, L1 or L2 in FIG. 18) may be set to a predeterminedvalue.

Additionally, in the example of FIG. 19, an exemplary format in whichthe response information of the CA-RSP frame is composed of a responsetype field and a body part is shown. The length of the response typefield may have a length of 3 bits. In this case, if the responseinformation part has a length of 1 octet, the body part has a length of5 bits and, if the response information part has a length of 2 octets,the body part has a length of 13 bits.

In the example of FIG. 20, the body part of the response information maybe composed of channel access duration information (FIG. 20(a)), channelaccess duration and timestamp information (FIG. 20(b)), current channelaccess group ID (CCAGID) and next access group offset (NAGSO)information (FIG. 20(c)), CCAGID, NAGSO, timestamp and granted channelaccess duration (GCAD) information (FIG. 20(d)) or new group ID (or newAID) information (FIG. 20(e)).

The information included in the second frame (or ACK, access control orCA-RSP frame) is exemplary and may be a combination of at least one ofthe various examples of FIGS. 18 to 20.

Hereinafter, the embodiments of the present invention using the firstframe and the second frame will be described in detail.

FIGS. 21 to 34 are diagrams showing examples of improved channel accessoperation using first and second frames proposed by the presentinvention.

In the example of FIG. 21, STA3 having GID 3 may transmit the firstframe (e.g., the CA-REQ frame) to the AP without receiving the beaconframe after waking up. That is, STA3 may transmit the CA-REQ frame tothe AP even at timing other than the channel access interval of thegroup thereof. In response thereto, the AP may transmit the second frame(e.g., the CA-RSP frame) to STA3. MD=1 may be set in the second frame toindicate, to STA3, that DL data to be transmitted is present.

In addition, the second frame may include timestamp information (seeFIG. 18(a)). STA3 may accurately perform time synchronization with theAP via the timestamp value included in the second frame. The STA whichhas performed time synchronization may substantially accurately computenext beacon transmission timing and operate in the sleep mode up to thattiming. STA3 which wakes up at next beacon transmission timing maysuccessfully receive the beacon to acquire current group information(that is, information indicating a group, channel access of which isgranted). Based on the acquired current group information, STA3 maycompute when channel access of the group thereof is granted. Forexample, STA3 which has confirmed that the channel access interval ofthe group thereof is after a third beacon frame may be switched to thesleep mode for power saving, wake up at third beacon timing to attemptchannel access, and receive DL data from the AP.

In the example of FIG. 22, as operation added to the example of FIG. 21,the STA computes the channel access interval of the group thereof fromthe timestamp information included in the second frame from the AP andthen wake up at corresponding timing to additionally perform the channelaccess process (e.g., transmission and reception of the CA-REQ andCA-RSP) before receiving DL data.

In the additional channel access process, the AP may transmit the secondframe (see FIG. 18(b)) including ACK information to the STA in responseto the first frame and then transmit data to the STA.

The ACK information included in the second frame may be defined in theform of the ACK field as shown in FIG. 18(b) or the MD field of theframe control field may be set to 1 in the various examples (see FIGS.18 to 20) of the second frame as in the ACK frame.

In addition, if the AP receives the first frame from the STA and channelaccess of the STA is possible, the AP does not transmit the CA-RSP frameas in the example of FIG. 21 but transmits the ACK frame to the STA toindicate that channel access is granted. In this case, if buffered DLdata to be transmitted to the STA is present, the MD field of the ACKframe may be set to 1.

In the example of FIG. 23, the AP which has received the first framefrom the STA may immediately transmit DL data to the STA withouttransmitting the ACK frame or the second frame including ACK. In theexample of FIG. 23, operation of the STA and the AP before GID 3 channelaccess interval is equal to that of FIG. 22. When the STA transmits thefirst frame (e.g., the CA-REQ frame) to the AP at the channel accessinterval thereof, the AP may immediately transmit DL data.

In the example of FIG. 24, the AP which has received the first framefrom the STA may transmit the second frame (see FIG. 18(c)) includingCASO and GCAD to the STA.

CASO indicates information regarding when channel access of the STAstarts to be granted. That is, CASO indicates information indicatingwhen an STA, the channel access of which is not currently granted, wakesup again after entering the sleep state again. In addition, CASO may beprovided as predetermined timer information (that is, the STA may wakeup again when a predetermined timer expires). In case of DL datatransmission, CASO may be set to a time when the AP transmits DL data tothe STA and, in case of UL data transmission, CASO may be set to a timewhen UL data transmission from the STA to the AP is granted.Alternatively, CASO may indicate a next target beacon transmission time(TBTT), a beacon transmission start time of a group, to which an STAbelongs, or a start time of a group channel access interval of an STA.In case of DL data transmission, the AP may attempt DL data transmissionto the STA after the time indicated by CASO. In addition, CASO may beset as a start location of a channel access interval (in the example ofFIG. 24, GID 3 channel access interval) of the group, to which the STAbelongs, regardless of DL/UL data transmission.

GCAD indicates a channel access duration, and a reference time (or astart time) thereof is CASO.

The STA which has acquired CASO and GCAD information from the AP via thesecond frame may attempt data reception at the time indicated by CASOeven when the group, to which the STA belongs, is not confirmed or thelocation of the channel access interval of the group is not confirmed(e.g., even when a separate beacon frame is not received). Accordingly,the STA which has received CASO and GCAD via the second frame may bemaintained in the sleep state up to the time indicated by CASO, therebyachieving additional power saving.

FIG. 24 shows the case in which the MD bit of the second frame is set to1 such that the STA wakes up at the time indicated by CASO and attemptsDL data reception during GCAD. Although not shown, if the MD bit of thefirst frame transmitted by the STA is set to 1, the STA may wake up atthe time indicated by CASO and attempt UL data transmission during GCAD.

In the example of FIG. 25, the AP which has received the first framefrom the STA may transmit the second frame (see FIG. 18(d) includingCAGN and NAGSO information to the STA.

The STA may confirm a group, channel access of which is currentlygranted, from the CAGN information. In addition, the STA may confirm thestart time of the next channel access interval from the NAGSOinformation. In the example of FIG. 25, the STA may confirm the channelaccess interval of the current group 1 (that is, GID 1) from the CAGNinformation received via the second frame and the start time of thechannel access interval of group 2 (that is, GID 2), channel access ofwhich is next granted, from the NAGSO information.

If it is assumed that the STA knows the total number of groups, thechannel access intervals of all groups have the same time length and theconfiguration information of the groups is the same, the STA may computethe location of the channel access interval of the group thereof usingCAGN and NAGSO.StartOffset(GID)=NAGSO+(|GID−CAGN−1|mod N)*I  Equation 1

In Equation 1, GID denotes the group number assigned to the STA and GIDmay be expressed as the access group number of the STA (AGNS). CAGNdenotes the number of a group, access of which is currently granted, asdescribed above. NAGSO denotes the start time of the channel accessinterval of the next access group as described above and may be definedin μs. N denotes the total number of groups supported by the AP. Idenotes the length of the channel access interval of one group. ∥denotes an absolute value and mod denotes modulo operation.

In the example of FIG. 25, since the total number of groups is 3, theGID of the STA is 3 and the current group number is 1, the start time ofthe channel access interval of the group (that is, group 3), to whichthe STA belongs, may be NAGSO+((3-1-1) mod 3)*I=NAGSO+I.

In Equation 1, it is assumed that, if the total number of groups is 4,the group numbers are 1, 2, 3 and 4. If the group numbers are 0, 1, 2and 3, Equation 1 may be changed to Equation 2 below.StartOffset(GID)=NAGSO+(|GID−CAGN|mod N)*I  Equation 2

If the AP provides the CASO and NAGSO information to the STA via thesecond frame, the STA may wake up at the start time of the channelaccess interval of the group thereof and receive data, without waking upthe next beacon timing to receive the beacon frame.

In addition, the STA should know the total number (N) of groups and thelength (I) of the group channel access interval in advance, in order toaccurately determine the start time of the channel access interval ofthe group thereof from the CASO and NAGSO information provided via thesecond frame. Information about the values N and I may be received fromthe AP when the STA is associated with the AP and the AID (or GID orPID) is assigned.

In the example of FIG. 26, if information about the total number (N) ofgroups managed by the AP and/or information about the length (I) of onegroup channel access interval is not received from the AP when the STAis assigned the AID (or GID/PID) or before the STA operates in the sleepmode, the AP may include and provide the information (e.g., the values Nand I) in the second frame along with CAGN and NAGSO. The STA maydetermine the start time of the channel access interval corresponding tothe group thereof.

In addition, in the method of including CASO and GCAD in the secondframe (e.g., the CA-RSP frame) (see FIG. 18(c)) or the method ofincluding CAGN and NAGSO (see FIG. 18(d), although the STA may performDL data reception from the AP without the channel access operation(e.g., first frame transmission) at the channel access interval assignedto the group thereof in the examples described with reference to FIGS.24 to 26, the present invention is not limited thereto. That is, asdescribed with reference to FIGS. 21 to 23, even in the method usinganother exemplary format of the second frame, when the STA attemptschannel access at the channel access interval thereof, first frametransmission, second frame reception, data reception (similar to theexample of FIG. 22), first frame transmission, ACK frame reception anddata reception (similar to the example of FIG. 22) or first frametransmission and data reception (similar to the example of FIG. 23) maybe performed.

In the example of FIG. 27, when the AP transmits the second frame (e.g.,the CA-RSP frame or the ACK frame) in response to the first frame (e.g.,the CA-REQ or PS-Poll frame) received from the STA, if the AP has datato be transmitted to the STA, the MD bit of the second frame may be setto 1 and indicated to the STA. Alternatively, the AP may immediatelytransmit data to the STA without transmitting ACK after receiving thefirst frame (e.g., PS-Poll).

In the example of FIG. 28, when the STA transmits the first frame (e.g.,the CA-REQ or PS-Poll frame) to the AP, if the STA has data to betransmitted to the AP, the MD bit of the first frame may be set to 1 andindicated to the AP. Alternatively, the STA may transmit the PS-Pollframe and then transmit data to the AP if receiving ACK or data from theAP.

FIGS. 27 and 28 show the cases in which the first frame transmitted fromthe STA, which does not receive the beacon frame immediately afterwaking up, to the AP and the second frame which is a response theretoare respectively CA-REQ and CA-RSP and the first and second frames usedfor A channel access at the channel access interval of the group, towhich the STA belongs, are respectively PS-Poll and ACK frames. Thepresent invention is not limited thereto and various modifications, suchas the case in which the first frame used by the STA, which does notreceive the beacon frame immediately after waking up, for channel accessis a PS-Poll frame and the case in which the first frame transmitted tothe AP at the channel access interval thereof is CA-REQ, are possible.

Although, in the examples of FIGS. 27 and 28, the STA, which does notreceive the beacon frame immediately after waking up, transmits thefirst frame for channel access and receives the second frame in responsethereto to acquire information about the channel access interval of thegroup, to which the STA belongs, similarly to FIG. 25, the presentinvention is not limited thereto. That is, examples of the first andsecond frames first exchanged with the AP after the STA is switched fromthe sleep mode to the awake mode may be configured according to any oneformat of FIG. 17 and FIG. 18.

In the example of FIG. 29, if the STA is switched from the sleep mode tothe awake mode in order to transmit a UL frame (e.g., a data frame, acontrol frame, a management frame, etc.), the STA may transmit the firstframe to the AP even when the beacon frame is not received. In thiscase, the STA may set the MD bit of the frame control field of the firstframe to 1 and transmit the first frame in order to inform the AP thatdata to be transmitted in UL is present. Additionally or alternatively,as in the example of FIG. 17(c), UL/DL indicator information may beincluded in the first frame so as to explicitly indicate whether thefirst frame (e.g., the CA-REQ frame) transmitted by the STA is a channelaccess request for DL reception or a channel access request for ULtransmission.

The AP which has received the first frame from the STA may extract GIDinformation of the STA from the AID information included in the firstframe. Based on this information, the AP may determine whether channelaccess of the STA is granted. For example, the AP may determine whetherchannel access of the STA is granted when the first frame of the STA isreceived. In the example of FIG. 29, since a time when the AP receivesthe first frame from the STA is a channel access interval for group 1(that is, GID 1) and the GID of the STA extracted by the AP from the AIDinformation of the first frame is 1, the AP may determine that channelaccess of the STA is granted. Thus, the AP may include informationindicating whether channel access of the STA is granted in the secondframe and transmit the second frame to the STA.

For example, the second frame may be configured in the format shown inFIG. 19. That is, the fields added to the basic fields (e.g., the framecontrol field, the AID field, the BSSID field and the FCS field) of thesecond frame may be collectively referred to as a response informationfield and the response information field includes a response type fieldand a body part. For example, the response type field may have a size of3 bits. The value of the response type field may indicate whetherchannel access of the STA is granted as described above. For example,the value of the response type field in the response information fieldof the second frame may be defined to have the meanings shown in Table 1below.

TABLE 1 Value of response type field Meanings Description 000 AcceptChannel access of STA is accepted 001 Reject Channel access of STA isrejected 010 GID/AID New GID (or AID) is assigned to STA reassignmentwhile channel access of STA is accepted 100-111 reserved For future use

As described in Table 1, if the AP accepts channel access of the STA(for example, if the GID for the GID channel access interval matches theGID assigned to the STA), the response type field in the second framemay be set to 000 and the second frame may be transmitted to the STA.

In addition, the value 000 of the response type field may mean ACK forthe request (channel access request using CA-REQ or PS-Poll) of the STA.Accordingly, if the STA, which has received the second frame in whichthe response type is set to 000, may perform UL channel access if theSTA has UL data to be transmitted to the AP (for example, if the STAsets the MD bit in the first frame to 1 and transmits the first frame tothe AP) and wait for and receive DL data from the AP if the STA has DLdata to be received from the AP (e.g., if the AP sets the MD bit in thesecond frame to 1 and transmits the second frame to the STA).

The second frame transmitted by the AP may include information aboutcurrent channel access duration along with the response type field (seeFIG. 20(a)). The STA may perform channel access during the currentchannel access duration and stop channel access if the channel accessduration ends.

The second frame may include timestamp information such that the STAaccurately performs time synchronization with the AP (see FIG. 20(b)).The timestamp information may be defined to be included only if thevalue of the response type is 000 (that is, channel access acceptance).Alternatively, in order to provide accurate time synchronization to theSTA regardless of the value of the response type, the timestampinformation may be always included in the second frame.

The AP may reject the channel access request of the STA. For example, ifthe GID corresponding to the current channel access interval does notmatch the GID of the STA or if the number of STAs which attempt currentchannel access is too large, the channel access request of the STA maybe prohibited. In this case, the AP may set the response type to 001 andtransmit the second frame to the STA.

In addition, the body part of the second frame may include a currentchannel access group ID (CCAGID) and a next access group start offset(NAGSO) field (see FIG. 20(c)). The CCAGID corresponds to theabove-described CAGN information. This example may be more suitably usedif the response type is 001 (that is, channel access rejection). Forexample, the STA, channel access of which has been rejected, may computethe start location (or the start offset) of the channel access intervalof the group, to which the STA belongs, based on Equation 1 or 2 fromthe CCAGID and NAGSO information included in the second frame. Thus, theSTA may attempt channel access at the start time of the channel accessinterval of the group, to which the STA belongs.

The body part of the second frame may include a channel access startoffset (CASO) and a granted channel access duration (GCAD) field (notshown), instead of the CCAGID and the NAGSO. This may be more suitablyused if the response type is 001 (that is, channel access rejection).The STA which has acquired the CASO and GCAD information from the AP viathe second frame may attempt data reception during the GCAD from thetime indicated by the CASO.

The body part of the second frame may include timestamp and GCAD fieldsin addition to the CCAGID and the NAGSO (see FIG. 20(d)). This may bemore suitably used if the response type is 001 (that is, channel accessrejection).

In the additional examples of the second frame format described withreference to FIGS. 19 and 20, at least one of timestamp, CCAGID, NAGSO,CASO and GCAD may be defined to be always included in the second frameregardless of the value of the response type (that is, channel accessacceptance). Thus, the STA may more accurately compute the channelaccess interval of the group, to which the STA belongs.

In addition, the AP may change the group, to which the STA belongs, togrant data transmission to the STA. In this case, the response type ofthe second frame may be set to 010 and a new GID (or a new AID) may beincluded in the body part. For example, although GID X of the STA whichrequests channel access (or transmits the first frame) does not matchGID Y of the current channel access interval, the AP may determine thatchannel access of the STA is granted in consideration of the currentnetwork situation (e.g., if the density of the access STA of the currentgroup is low or if the STA which requests channel access is in anemergency state). In this case, the AP may reassign Y to the STA as anew GID such that the STA performs channel access at the current channelaccess interval. If the GID is not separately set but is estimated frominformation included in the AID, the AP may include a new AID in thesecond frame and transmit the second frame to the second frame, therebyassigning the new AID.

The STA which has received the second frame in which the response typeis 001 (that is, channel access of the STA is not granted at the channelaccess interval corresponding to the current time) may compute thechannel access grant time and duration thereof using the informationincluded in the second frame. The STA may be in the sleep state up tothat time and then switched to the awake state at that time, therebyattempting channel access.

In case of DL channel access, the STA may wait for DL data receptionfrom the AP after the time determined by the computation result.Alternatively, for DL channel access, the STA may request channel accessfrom the AP at the time determined by the computation result(transmission of the first frame (the CA-REQ or PS-Poll frame)). Inresponse thereto, the AP may transmit the second frame (e.g., CA-RSP,ACK, etc.) to the STA and then transmit DL data or immediately maytransmit DL data to the STA without second frame transmission.

In case of UL channel access, the STA may request UL channel access(e.g., transmission of the first frame (e.g., CA-REQ or RTS)) or maytransmit UL data without first frame transmission at the time determinedby the computation result. The AP may transmit the CA-RSP frame or theACK frame if receiving the CA-REQ frame from the STA. Alternatively, theAP may transmit CTS if receiving RTS from the STA. Alternatively, the APmay transmit ACK if receiving data from the STA without the first frameor RST.

In the examples of FIGS. 30 to 34, the method using PS-Poll frame as thefirst frame proposed by the present invention will be described. ThePS-Poll frame means a frame transmitted from the STA to the AP so as toperform the DL channel access process (that is, a process of confirmingwhether the AP has data to be transmitted to the STA). In responsethereto, the AP may transmit data or transmit ACK in which MD=1 is setand then transmit data, if the data to be transmitted to the STA ispresent. If data to be transmitted to the STA is not present, the AP maytransmit ACK, in which MD=0 is set, to the STA.

An existing PS-Poll frame may include a purpose for informing the APthat the STA which has received the TIM included in the beacon frame isawake and is ready for receiving DL data, as described with reference toFIG. 10 or FIG. 11. However, although the format of the PS-Poll frameused in the present invention (that is, the PS-Poll frame as the exampleof the first frame) may be equal to that of the existing PS-Poll frame,the PS-Poll frame may be transmitted by even the STA which does notreceive the beacon frame (that is, does not receive the TIM).

In the example of FIG. 30, the STA in the sleep state (or the dozestate) may be switched to the awake state to transmit the first frame(e.g., the PS-Poll frame) to the AP in order to confirm whether DL datato be transmitted thereto is present. In response thereto, the AP maytransmit, to the STA, the second frame (e.g., the ACK frame) indicatingthat data to be transmitted to the STA is present (that is, MD=1 isset). Thereafter, the AP may transmit DL data to the STA and the STA maytransmit ACK to the AP in response thereto.

In the examples of the present invention, when the AP transmits, to theSTA, the second frame (that is, the ACK frame or response frame as theresponse to the channel access request (or PS-Poll) of the STA), the STAmay include information (e.g., time synchronization information)directly/indirectly indicating the channel access interval of the group,to which the STA belongs. For example, if the STA operates in thelong-sleep mode, since a probability that time synchronization with theAP is not performed when the STA wakes up and transmits the first frameis high, the AP may include timestamp information in the second frameand transmit the second frame. That is, according to the presentinvention, although the STA does not solicit the timestamp from the AP,the AP may transmit the unsolicited timestamp information to the STA.

The time synchronization information (e.g., the timestamp information)of the STA may be included in the above-described second frame (e.g.,the CA-RSP frame, the ACK frame, etc.) or may be transmitted in a stateof being piggybacked on the data frame if data is transmitted withoutACK.

In the example of FIG. 31, the STA having PID (page ID)=2 may transmitthe first frame (e.g., PS-Poll) to the AP. The AP may set MD=1 toindicate that data to be transmitted to the STA is present, whiletransmitting ACK in response to the PS-Poll frame. While the AP preparesdata to be transmitted to the STA, the time synchronization information(e.g., the timestamp information) may be transmitted to the STA. As theframe via which the timestamp information is transmitted, the example ofthe second frame (e.g., the CA-RSP frame format of FIG. 18(a)) may beused. Alternatively, in the example of FIG. 31, the timestampinformation may be transmitted in a standalone manner using a separateframe (e.g., an access control frame). The access control frame mayinclude information (e.g., the response type field) indicating whetherthe channel access request of the STA is granted. In the example of FIG.31, since the PID of the STA is equal to the PID of the current channelaccess interval, channel access is granted. Accordingly, the accesscontrol information may include information indicating channel accessacceptance and timestamp information. The STA which has received thetimestamp information may adjust the time thereof based on the timestampinformation. Thereafter, the AP may transmit data to the STA and the STAmay transmit ACK in response thereto and operate in the sleep modeagain. Alternatively, the second frame may be transmitted in a state ofbeing concatenated with the data frame.

Since the AP may implicitly confirm whether the STA operates in thelong-sleep mode from the AID of the STA, it may be determined whetherthe time synchronization information (e.g., the timestamp information)of the STA is provided to the STA. In addition, when the STA transmitsthe first frame (e.g., the CA-REQ or PS-Poll frame) to the AP, the STAmay include information explicitly indicating whether the STA operatesin the long-sleep mode. If the AP confirms that the STA operates in thelong-sleep mode from the above information, the time synchronizationinformation (e.g., the timestamp information) may be provided to theSTA.

Although the timestamp information is described as the timesynchronization information of the STA in the above examples, thepresent invention is not limited thereto. That is, the STA may providethe STA not only with the information for directly/indirectly adjustingtime synchronization but also with a variety of information shown inFIGS. 18 to 20. For example, information about a time offset (e.g.,CASO) at which the STA wakes up may be provided via the second frame(e.g., the CA-RSP frame, the ACK frame, etc.).

In the example of FIG. 32, if the STA having PID=1 transmits the firstframe (e.g., the PS-Poll frame) and the AP has data to be transmitted tothe STA, the AP may transmit the ACK frame, in which MD=1 is set, inresponse to the PS-Poll frame. However, since the current channel accessinterval is for the STA corresponding to PID=2, channel access of theSTA having PID=1 is not granted. Accordingly, the AP may transmit thesecond frame (e.g., the access control frame) including informationindicating that channel access is rejected to the STA. In addition, theAP may additionally include information about the start offset (or thestart time) in the access control frame, such that the STA operates inthe sleep state up to the channel access interval for the PID, to whichthe STA belongs. Then, the STA may wake up at the time indicated by thestart offset value to perform channel access operation (e.g.,transmission of the first frame (e.g., the PS-Poll frame) and properlyreceive DL data thereof.

In the example of FIG. 33, if the STA having PID=1 transmits the firstframe (e.g., the PS-Poll frame) and the AP has data to be transmitted tothe STA, the same operation as FIG. 32 is performed. However, in orderto simplify the channel access request/response procedure, although datafor STA is present, if the PID of the STA does not match the PID of thecurrent channel access interval, the AP may set the MD bit of the secondframe (e.g., the ACK frame) to 0. In addition, information about whenthe channel access interval for the PID, to which the STA belongs,starts (that is, the start offset) may be provided to the STA along withthe ACK. Various formats of the second frame (for example, the CA-RSPframe, the ACK frame, the access control frame, etc.) may be used.

In the above examples, the STAs, which have received information aboutthe channel accessible time from the AP, may operate in the sleep modeand then wake up at that time to attempt channel access. In this case,the woken-up STA may transmit the first frame (e.g., the CA-REQ orPS-Poll frame) or wait for DL data without transmitting the first frame.

In the example of FIG. 34, if the STA having PID=1 transmits the firstframe (e.g., the PS-Poll frame) and the AP has data to be transmitted tothe STA, the same operation as FIG. 33 is performed. However, in thepresent example, the AID (or PID/GID) may be reassigned to the STA whichdoes not belong to the PID of the current channel access interval togrant channel access at the current channel access interval, therebysimplifying the operations of the STA and the AP. In the example of FIG.34, the AP which has received the first frame of the STA having PID=1may transmit information for reassigning 2 which is the PID of thecurrent channel access interval as a new PID of the STA having PID=1.The above information may be used using the exemplary format (e.g., FIG.20(e)) of the second frame or a new independent frame (e.g., the accesscontrol frame). In this case, the response type field of the secondframe may be set to a value indicating AID (or PID/GID) reassignment,instead of a value indicating channel access acceptance or rejection.Thus, the STA may re-set the AID (or PID/GID) to a new value, performchannel access to the AP at the current channel access interval, andproperly receive DL data.

In addition, even if the AP reassigns the AID (or PID/GI) of the STA,the time synchronization information (e.g., the timestamp information,information about the time offset at which the STA wakes up (e.g.,CASO), channel access duration information, etc.) of the STA may beadditionally provided to the STA. That is, as described above, the APmay include the time synchronization information of the STA in thesecond frame regardless of channel access acceptance/rejection/IDreassignment of the STA for requesting channel access (that is,transmitting the first frame) at the current channel access interval (atany time). In addition, the time synchronization information may includethe timestamp information, next beacon transmission time information,information about the time offset at which the STA wakes up (e.g.,CASO), information about duration when the STA may use the channel(e.g., GCAD), etc.

FIG. 35 is a flowchart illustrating a channel access method according toan example of the present invention.

In step S3510, a first STA (e.g., a non-AP STA) may be switched from thesleep state to the awake state.

In step S3520, the first STA may transmit the first frame (e.g., thePS-Poll or CA-REQ frame) proposed by the present invention to a secondSTA (e.g., an AP). For example, if the first STA wakes up, the first STAmay transmit the first frame at any time (e.g., even when a TIM is notacquired via a beacon frame).

In step S3530, the second STA may determine whether channel access ofthe first STA is granted in consideration of the information included inthe first frame transmitted by the first STA, a group numbercorresponding to a current channel access interval, and a network state.

In step S3540, the second STA may transmit the second frame (e.g., ACK,CA-RSP or access control frame) proposed by the present invention to thefirst STA in response to the first frame received from the first STA.The second frame may include timing information such as timesynchronization information of the first STA or information about whenthe STA wakes up after entering the sleep mode. The informationdescribed in the above-described examples of the present invention maybe included in the second frame.

Therefore, the first STA may perform channel access while minimizingpower consumption, although time synchronization with the second STA isnot performed.

Details described in the above embodiments of the present invention maybe independently applied or two or more embodiments may besimultaneously applied.

FIG. 36 is a block diagram showing the configuration of a wirelessapparatus according to one embodiment of the present invention.

The AP 10 may include a processor 11, a memory 12 and a transceiver 13.The STA 20 may include a processor 21, a memory 22 and a transceiver 23.The transceivers 13 and 23 may transmit/receive a radio frequency (RF)signal and implement a physical layer according to an IEEE 802 system,for example. The processors 11 and 21 may be respectively connected tothe transceivers 13 and 21 to implement a physical layer and/or an MAClayer according to the IEEE 802 system. The processors 11 and 21 may beconfigured to perform operation according to the above-described variousembodiments of the present invention. In addition, modules implementingoperations of the AP and the STA according to the above-describedembodiments of the present invention may be stored in the memories 12and 22 and may be executed by the processors 11 and 21, respectively.The memories 12 and 22 may be mounted inside or outside the processors11 and 21 to be connected to the processors 11 and 21 by known means,respectively.

The detailed configuration of the AP and the STA apparatus may beimplemented such that details described in the above embodiments of thepresent invention is independently applied or two or more embodimentsare simultaneously applied. In this case, overlapping details will beomitted from the description for clarity.

The above-described embodiments of the present invention can beimplemented by a variety of means, for example, hardware, firmware,software, or a combination of them.

In the case of implementing the present invention by hardware, thepresent invention can be implemented with application specificintegrated circuits (ASICs), Digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), a processor, a controller, amicrocontroller, a microprocessor, etc.

If operations or functions of the present invention are implemented byfirmware or software, the present invention can be implemented in theform of a variety of formats, for example, modules, procedures,functions, etc. The software codes may be stored in a memory unit sothat it can be driven by a processor. The memory unit is located insideor outside of the processor, so that it can communicate with theaforementioned processor via a variety of well-known parts.

The detailed description of the exemplary embodiments of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the exemplary embodiments, those skilled in the artwill appreciate that various modifications and variations can be made inthe present invention without departing from the spirit or scope of theinvention described in the appended claims. For example, those skilledin the art may use each construction described in the above embodimentsin combination with each other. Accordingly, the invention should not belimited to the specific embodiments described herein, but should beaccorded the broadest scope consistent with the principles and novelfeatures disclosed herein.

INDUSTRIAL APPLICABILITY

Although the above-described various embodiments of the presentinvention are applied to the IEEE 802.11 system, the embodiments of thepresent invention are applicable to various mobile communicationsystems.

The invention claimed is:
 1. A method of performing channel access at aspecific station (STA) in a wireless communication system, the methodcomprising: receiving, at the specific STA, a beacon frame includinginformation on a channel access interval assigned to each group ofstations, wherein the specific STA is allowed to access a channel withina channel access interval assigned to a specific station group includingthe specific STA; transmitting, at the specific STA, a power save(PS)-Poll frame to an access point (AP); receiving, at the specific STAfrom the AP, a response frame in response to the PS-Poll frame, theresponse frame including a channel access start offset value for thespecific STA, and determining a channel access interval for the specificSTA based on the channel access start offset value included in theresponse frame.
 2. The method according to claim 1, wherein the value ofthe channel access start offset determines when the specific STA wakesup again after entering a sleep state.
 3. The method according to claim1, wherein the PS-Poll frame includes at least one of an associationidentifier (AID) of the specific STA, a group ID assigned to thespecific STA or uplink (UL)/downlink (DL) channel access requestindicator information.
 4. The method according to claim 1, wherein theresponse frame includes at least one of a timestamp, acknowledgement(ACK), granted channel access duration, a current access group number, acurrent channel access group identifier, a next access group startoffset, a channel access start offset or a new STA identifier.
 5. Themethod according to claim 4, wherein the channel access start offset isone of a downlink data reception start time of the specific STA, a starttime of a channel access interval at which uplink data transmission ofthe specific STA is granted, a next beacon transmission time, a nextbeacon transmission start time for the station group, or a channelaccess start time of a group assigned to the specific STA.
 6. The methodaccording to claim 1, wherein: the response frame includes response typeinformation, and the response type information indicates one of channelaccess allowance, channel access rejection or STA identifierreassignment.
 7. The method according to claim 1, wherein the PS-Pollframe is transmitted if the specific STA is switched from a sleep stateto an awake state.
 8. The method according to claim 1, wherein, withoutreceiving a beacon including a traffic indication map (TIM),transmission of the PS-Poll frame is allowed in an awake state of thespecific STA.
 9. The method according to claim 1, wherein whetherchannel access is granted is determined by the AP based on whether apage identifier of the specific STA matches a page identifier related toa current channel access interval.
 10. The method according to claim 1,wherein the transmitting the PS-Poll frame to the AP is performed duringa channel access interval not assigned to the specific station group.11. A method of supporting channel access of a station (STA) in anaccess point (AP) of a wireless communication system, the methodcomprising: transmitting a beacon frame including information on achannel access interval assigned to each group of stations, wherein aspecific STA is allowed to access a channel within a channel accessinterval assigned to a specific station group including the specificSTA; receiving a power save (PS)-Poll frame from the specific STA; andtransmitting, to the specific STA, a response frame in response to thePS-Poll frame, the response frame including a value of a channel accessstart offset value for the specific STA, wherein the specific STA usesthe channel access start offset value to determine a channel accessinterval for the specific STA.
 12. A station (STA) apparatus forperforming channel access in a wireless communication system, the STAapparatus comprising: a transceiver; and a processor, wherein theprocessor is configured to control the transceiver to: receive a beaconframe including information on a channel access interval assigned toeach group of stations, transmit a power save (PS)-Poll frame to anaccess point (AP) and receive, from the AP, a response frame in responseto the PS-Poll frame, the response frame including a value of a channelaccess start offset value for the STA, and wherein the processor isfurther configured to determine a channel access interval for the STAbased on the channel access start offset value included in the responseframe.
 13. An access point (AP) apparatus for supporting channel accessof a station (STA) in a wireless communication system, the AP apparatuscomprising: a transceiver; and a processor, configured to control thetransceiver to: transmit a beacon frame including information on achannel access interval assigned to each group of stations, receive apower save (PS)-Poll frame from a specific STA and transmit, to thespecific STA, a response frame in response to the PS-Poll frame, theresponse frame including a value of a channel access start offset forthe specific STA, and wherein the specific STA uses the channel accessstart offset value to determine a channel access interval for thespecific STA.